<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .asda{
      color: red;
    }
    div span{
      color: blue;
      font-size: 50px;
    }
    button:hover{
      background-color: red;
      font-size: 30px;
    }
    button:active{
      background-color: palegreen;
      font-size: 50px;
    }
  </style>
</head>
<body>
  <p>段落1</p>
  <p class="asda">段落2</p>
  <p>段落3</p>
  <span>外面的span</span>
  <div>
    <span>里面的span</span>
  </div>
  <button>按钮</button>
</body>
</html>